The present invention relates to a device for electrically monitoring oil level, particularly in automobile internal combustion engines, which device is housed within a hollow space in an oil measurement rod and is located at the height of a filling level, the dropping of the oil level below the filling level being signaled and for which purpose is connected to an evaluation circuit having a source of current, the evaluation circuit switching at at least one switching threshold, there being a second temperature-dependent resistance element which is positioned in the oil measurement rod at a lower place than the first temperature-dependent resistance element, as part of an oil-temperature display device.
In such known devices, a resistor having a positive temperature coefficient (PTC) which can be encapsuled in bead form in glass, is used, for instance, as a temperature-dependent resistance element. The temperature-dependent resistance element can be connected via lead wires to an evaluation-circuit with a source of current, by which circuit the temperature prevailing at the resistance element can be converted into an electrical signal. In this connection, use is made of the fact that the resistance element, depending on how far it is immersed in a medium, typically oil of better thermal conductivity than air, gives off to its environment a greater or lesser amount of the heat which is produced by a current flowing through the resistance element. If for instance, the resistance element is immersed in oil, a substantial amount of heat is given off and the temperature of the resistance element is correspondingly low. To this there corresponds a relatively low resistance and small voltage drop over the temperature-dependent resistance element or a large voltage drop over an ohmic resistance element connected in series therewith. On the other hand, if the temperature-dependent resistance element is in air, then only a small amount of heat is given off and the resistance element is heated to a comparatively high temperature.
As a result of the fact that the resistance of the temperature-dependent resistance element is then higher, there is a relatively large voltage drop over it and there is a corresponding voltage drop which can be evaluated over the series-connected ohmic resistor. The evaluation-circuit is provided for this purpose and has a bridge circuit in which the temperature-dependent resistor is connected in one branch of the bridge, and control connections for an amplifier, in particular a transistor which switches at at least one switching threshold, are connected in the neutral branch thereof. The amplifier switches at a switching threshold in particular, when the oil level drops below a predetermined amount. With a somewhat higher switching threshold, switching can again take place when the oil level rises above the level at which the first switching took place. Thus a drop below a predetermined amount of oil in an internal combustion engine can be signaled, for which purpose a light-emitting diode can be used.
However, these known devices for the electrical monitoring of oil level operate only within a relatively narrow temperature range unless special compensatory measures are taken. This is due to the fact that, with an increase in the surrounding temperature, for instance the temperature of the oil, the voltage drop over the ohmic resistor connected in series with the temperature-dependent resistor becomes constantly smaller. Also the difference in the voltage drops at the series-connected ohmic resistor between the operating cases of immersed and non-immersed temperature-dependent resistor decreases.
In order to extend the utility of the known devices for the electric monitoring of oil level to a larger temperature range it is also known to adapt the response level of the amplifier of the evaluation circuit as a function of temperature. For this temperature compensation a further temperature-dependent resistor can be so located that it is surrounded by the fluid to be monitored, namely the oil. However, the locating of the further temperature-dependent resistor within the limited hollow space of a hollow oil measurement rod affords difficulties, particularly if the oil measurement rod already contains a second resistance element which is part of a device for measuring the temperature of the oil.